Phototube



March 9, 1937.

H. NELSON PHOTOTUBE original Filed sept. 19, 1935 XNVENTOR HERBERT NELSON Patented Mar. 9i, 1937 PHOTOTUBEl Herbert Nelson, "Bloomfield, lN iJ., assignor to Radio Corporation of America, a corporation ofv Delaware original application september 19, 1933; serial f No. 690,045. yDivided -an'd this application August 26, 193s, serial No. 97,883

5 Claims.

This invention relates to spacefdischarge"v devices which respond to light, and morelparticularly such devices in Whchphotoelectric currents control. the magnitude of a thermionicdischarge.

5 This application isla division of'mycorresponding application-Serial No. 690,045, iiled' Septemberl 19,A 1933,- `and assigned` to` theRadio. Corporation of America.

' Phototubes as usually made are 1in most'of their practical applications used in conjunctionvwith a vacuum tube amplier. A-high resstancemust be used i-nl the external circuit ofthe photoelectrodes'topbtain from the small photoelectriclcurrent 'suicient voltage drop inl the resistance Ato aiectfthe'bias of the gridv of thef2ampler=to which' the phototube is coupled, .andft-hahigh impedanceof lthe phototube causes Ldiiicultyiin making an` efficient coupling between the 'phototube and the-amplier.

:2O 'l Tosi-mplify the problem Yof amplifying a photoelectric-current use hasbeen madeJof-a combinationltube comprising a phototubeanda vacuum tube Y amplifier mounted,v in-4 one lbulb, -f-withf the photocathode connected "to thefcontrolrgrid of thefampli-eryandf the photoanode connected-to theanode of thelamplier, -but this combination tubei has not been particularly successful commercially. The-structural arrangementis complicated-some kind of shieldingarra-ngement be- .30 tween phototube and amplifiervv is necessary, and al'vacuumtype phototube must be-used because thethermionic amplifier is of thevacuum type. Oneobject-of the present invention isfk to provide a method of photoel'ectrically controlling the output of athermioniccathode in a novelevvaiT whereby va greater output is obtained at -loWer 'voltages than by the method heretofore' available.

="Another objectfiis to provide a phototube in 40 whichthevoutput-of a thermionic cathode isicontrolled by a photocathode-in a simplean'dldi-rect way and Without the aid ofA a controlfgridwor ci high external resistance.

A further #object is 'to' provide a phototubefin which the current flow from afthermiomc-cathode in the same biilb as a photoelectrode is controlled by the etects produced' in the atmosphere ofthe Ybulb by-a` 1photoelectric current.

Still another object is to provide a highoutput Yphototube having avery simple and strongelectrode assembly.

Y In y accordance with my 'invention amplification Aof the photoelectric current is obtained by means of` a thermionic cathode such as a straightfilai* ment, a light sensitive photocathode adjacent the (Cl. Z50-275) thermionic cathode, and a grid-like electrode inter-posed betweenlthe two cathodes and lsurrounding the ilamentfand actingas. a' common anode for both'cathodes. :These three electrodes are mounted in a'tube containingyargon or similar y5 gas ata pressureof tok 125 microns-suchas is commonly used in gas type phototubes.

Inoperation the three electrodes are maintained at different potentialssochosen that the diierence of= potential t betweenthe grid anode 10 and the thermionic cathode is less than the ionizationfpotential -of the. gas, but the; potential difference between the anode and the photocathode is greater than the. ionization potential. y Under such conditions a.Y small change in the l5 emissionl of the z photocathode causes a large change 'in 'the thern'iionic`- current tothe anode. It is mybelef thatrwhen; the tube is in operation and as long as the photocathode is dark, the currentifrom thesthermionic cathode' tof the anode v20 is 'substantially free fromcionization and is space charge limited. When light falls on the tube a photocurrent flows from thephotocathode tof the anode and ionizes the gas -to an extent' dependent on themagntudeof 'thephoto'current '.Most. of 25 the v'ions appear near the anodewhere` the speed of fthe" photoelectrons is. highest, -fand hence rare in the vicinity of the thermionicf cathode. The positive ions partially'neutralize the space charge adjacentthe thermionic' cathode, Land.A as aresult :30 the thermionic i current? increases, andain effect alvlarge amplication =oi`-thexphotocurrent is obtained. i

The invention -Will be fbetter. understood by reference to the followings'pecicaticn When con- L35 sidered in connectionl with :the accompanying drawing vin which:

Figure lrfisfa perspective view of one form of tubefmade 'in 'accordance with my invention, parts being 'brokenf away better'to showinternal i4() structure;

'Fig-ure 2 is a perspective view ofia'lmodication offthe tube, parts being broken awayA better` to show internalstructure Figures 3` and'4 are Icircuitrdiagrams' of prac- =45 tical applications of the tube;

Figure 5 is-a curve showingthe relationfbetween photoelectric and thermioniclcurrents -inxthe tube.

The particular form-lof 'tube'sho-Wn ink Figure l has-a cylindricalelectrode-assemblyv compris- :50 ing a `th rmionic cathode l, preferably a'V shapedlament coated with oXidesf-suc'h as barium andstrontiumoxides. @The cathode emission is not critcaL-Jb'ut-fshouldzbe suillcientto cause the current to' be limitedlbyz space-"charge, 1,155

The cooperating tubular electrode 2, which is coaxial with and surrounds the thermionic cathode, operates as an anode, and is conveniently constructed like the usual grid consisting of a helix wound on two side rods, so that electrons may pass through it. This grid electrode 2 may be either flat or cylindrical, and the diameter and turns per inch of the wire wound on the side rods may be selected to give the tube the desired electrical characteristics. The grid-like or perforated anode 2 is surrounded by atubular photocathode 3, which is coaxial with the anode 2 and which may also be flat or cylindrical. The photocathode 3 may be made light sensitive on its outer surface in well known ways, and I have obtained good results with the process generally used to make thin lm caesium oxide photocathodes. I prefer to make the photocathode of silver coated sheet metal, and to permit operation of the tube independent of the direction of the incident light. I may perforate the photocathode with a plurality of openings 4. These openings 4 in the photocathode should be large enough so that the electrostatic field in the opening will not prevent electrons emitted from the outer surface of the photocathode passing through the opening to the anode 2. The filamentary thermionic cathode I is carried on leads 5 and 6, the anode 2 on leads 1 and 8, and the photocathode 3 on leads 9 and I0, all these electrode leads being carried by the stem which is sealed into the neck of a cylindrical bulb I2. The tubes should contain enough gas to produce appreciable ionization at reasonable operating voltages, such as to 100 volts. Good results have been obtained with tubes, made as shown in Figure 1, which contained argon at the usual pressure of gas type phototubes such as to microns.

The tube of Figure l is satisfactory for practical purposes, but for exceptionally accurate measurements the tube may be modified, as shown in Figure 2, to embody precautions to prevent the light from the filament affecting the photocathode. The tube shown in Figure 2 has a filament I3, of straight fine wire, such as nickel, coated with barium and strontium oxides. The upper end of the filament I3 is connected to a wire I4, and both filament I3 and the wire I4 are supported by lead wires I5 and I6. A cylindrical grid electrode or anode I1 having metal end collars I8 and I9 is mounted to surround and be coaxial with the cathode I3, and is supported by a lead wire 20 welded to the lower end collar I9. A at photocathode 2|, of solid sheet metal with its surface rendered light sensitive, is set edgewise to the lament I3, which lies in the plane of the iiat photocathode 2|. The only part of the photocathode exposed to the light from the filament I3 is the edge 22. Additional shielding of even the edge 22 of the photocathode may be obtained by a thin narrow metal strip 23 mounted on the metal collars I8 and I9 to lie in alignment with the filament I3 and to be out of contact with the wire coil or helix of the grid electrode or anode I1. This narrow strip shades or shields the photocathode 2| from the incandescent filament I3. In a tube thus constructed the photocathode 2l is affected only by light which is intentionally projected upon its surface to operate the tube. The photocathode 2| is carried by a lead Wire 24, and the lead wires for all electrodes are supported by the glass stem 25, which is sealed into the neck of the glass bulb 26 which encloses the entire Velectrode assembly. This tube operates in essentially the same way as the tube shown in Figure 1.

Tubes constructed as shown in Figures l and 2 have enough output to operate a relay when connected as shown in Figures 3 and 4, in which the tube elements are represented diagrammatically. In Figure 3 the lamentary thermionic cathode 21 is heated by a 1.5 volt lament battery 28, the perforated anode 29 is kept about 6 volts positive with reference to the cathode 21 by the 6 volt plate battery 30, and the photocathode 3| is made negative with reference to both the filamentary cathode 21 and the anode 29 by the 45 volt battery 32, which has its positive terminal connected to the filament 21 and also to the negative pole of the anode battery 30. With no light on the photocathode 3| of the tube connected as shown in Figure 3, there is no photoelectric current in the tube, but a thermionic discharge, too small to operate the relay 33, flows between the lamentary cathode 21 and the anode 29. As the Voltage drop of about 6 volts between the cathode 21 and the anode 29 is less than the ionization potential of the gas in the tube, the discharge between them is free from ionization and is limited by space charge.

When light falls upon the light sensitive cathode 3| photoelectrons are emitted by it and travel toward the anode 29 and the thermionic cathode 21, both of which are positive with reference to the photocathode. The voltage drop between the photocathode and the anode 29, due to the two batteries 30 and 32, is greater than the ionizing potential of the gas in the tube, hence the photoelectrons flowing to the anode 29 and filament 21 ionize the gas and thus produce positive ions. The number of positive ions thus produced is dependent on the number of photoelectrons emitted by the photocathode, and these positive ions neutralize to an extent dependent on their number the electron space charge between the fllament 21 and the anode 29. A comparatively small photoelectron current from the photocathode 3| will produce enough ions to have a marked effect on the space charge and thereby cause a comparatively large thermionic current to flow from the lament 21 to the anode 29 whereupon the relay 33 operates and the work circuit 34 is closed.

Figure 4 illustrates an adaptation to a picture transmission, television, or other like circuit of a .phototube embodying my invention and operating as explained in connection with Figure 3. The circuit of Figure 4 differs from that of Figure 3, in that, the l5 volt plate battery 35 is .of a higher potential than the plate battery 30 in Figure 3, and that therelay 33 in the anode circuit is replaced by a coupling transformer with its primary 30 in the anode circuit and its secondary 31 connected to the first tube ofv a thermionic amplifier. The output impedance of my photoamplifier tube is comparatively low, hence the impedance of the transformer is correspondingly low, and need not necessarily be higher than 2000 ohms. Such a low impedance transformer is much less expensive and much less susceptible to electrical disturbances such as pick-up from V of the light falling on the photocathode. In this tube a photoelectric current of one microampere corresponds to a thermionic current of approximately three milliamperes, as shown by the dotted line 39, and an increase in photoelectric current to three microamperes causes the thermionic current to increase to 4.2 milliamperes, as shown by the dotted line 4U. The ampliiication of the photoelectric current is so great that in many cases my tube will replace the usual combination of a phototube and glow tube, or phototube and external amplifier, and will produce much the same results by a simpler and l-ess expensive device.

It is to be understood that the embodiments of the invention hereinbefore described are merely illustrative and that many changes may be made without departing from the scope of the invention which is limited only by the appended claims.

I claimt 1. A phototube comprising an envelope containing gas at a reduced pressure and enclosing a thermionic cathode, a grid-like anode surrounding said cathode, and a perforated electrode surrounding said anode and exteriorly sensitized with light responsive electron emitting material.

2. A phototube comprising a gas lled envelope, a thermionic cathode, and a light sensitive photocathode surrounding said thermionic cathode, all points of the sensitive surface of the pho- 3 tocathode being shielded against iight and e1ectron bombardment from said thermionic cathode, and an anode to receive electrons from said thermionic cathode in the envelope.

3. A phototube comprising an envelope containing an inert gas at a reduced pressure and enclosing a thermionic cathode, an electron collecting electrode surrounding said thermionicl` cathode, a cylindrical electrode surrounding said collecting electrode and exteriorly sensitized with light responsive electron emitting material.

4. A phototube comprising an envelope containing an inert gas at a reduced pressure and enclosing a, thermionic cathode, a tubular anode surrounding said cathode, and means for ionizing said gas to reduce the space charge about said therrnionic cathode comprising a tubular sheetlike electrode sensitized with light responsive electron emitting material. c.

5. A phototube comprising an envelope containing gas at a reduced pressure and enclosing a thermionic cathode, an electron collecting electrode comp-rising a grid-like anode surrounding said cathode, and means for ionizing said gas between the anode and thermionic cathode comprising a. perforated tubular electrode surrounding said anode and exteriorly sensitized with light responsive electron emitting material.

HERBERT NELSON. 

